Waving when running is a bit weird, as you’re passing within a meter or two of the other runner. I mutter “How’s it going?” and move on. A personal greeting.

On the bike, normally the other cyclist is on the opposite side of the road, so a wave or nod is more appropriate. Greeting at a distance.

When approaching a runner/cyclist from behind is trickier. Do I say something and it might come off as smug in a “Quake at my awesome speed fellow earthling”, or nothing which could be interpreted as “I’m so good I don’t even acknowledge you”. I don’t think I can win (if there’s something there to be won) so I still mutter “How’s it going” and keep the head down.